An absolute impossibility; something that can never happen or is utterly beyond the realm of possibility.
他想在一天之内完成这个复杂项目，简直是马生角。
He wants to complete this complex project in one day, which is an absolute impossibility.
Era:
Ancient
Frequency:
Synonym:
Story:
During the late Warring States period, the Qin and Yan states outwardly maintained friendly relations, each sending a royal prince to the other as a hostage.
Prince Dan of Yan was a hostage in Qin, where King Ying Zheng treated him with great rudeness and disdain.
Prince Dan requested permission from King Zheng to return to Yan, but King Zheng retorted that he would only allow it when a horse grew horns and a crow turned white.
Prince Dan eventually escaped back to Yan and later dispatched Jing Ke to assassinate King Zheng.
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